Advances in automotive engine hardware are driving demand for higher performing lubricants and engineered solutions to the challenges that come with new engine design, and nowhere is this more true than TGDI – turbocharged gasoline direct injection. A group of industry experts addressed this technology and its impact on the lubricants industry during a July webinar co-hosted by SAE International and additive maker Lubrizol.
Turbocharged gasoline direct injection engines go by many acronyms: TGDI, TFGI and DIG-T, to name just a few. They can deliver high horsepower in a small package, along with improved fuel economy and reduced emissions – the Holy Grail for original equipment manufacturers dealing with government regulations.
The first direct-injected gasoline engines were developed over 100 years ago. They suffered from poor timing systems and never found a large place in the market. World War II brought turbocharged direct injection engines into more widespread use as part of the aircraft technology of all combatants.
Beginning in the mid-1990s, GDI became part of the new engine development cycle. Mitsubishi introduced its system in 1996, followed closely by Nissan and Toyota. These engines were primarily used in Europe, and soon Renault and Volkswagen brought their own systems into the marketplace. Ford, General Motors and others chimed in, too.
In a GDI engine, fuel is sprayed directly into the combustion chamber just prior to ignition (rather than being mixed with air outside the cylinder first). The combustion chamber is one of the hottest areas in the engine, and the fuel spray is vaporized almost instantly. The injection pressure also creates a more even distribution of fuel in the air charge, which results in more complete combustion than in previous engine designs. The result is better fuel efficiency.
Fuel combustion is limited, however, by the amount of air that is introduced into the combustion chamber. In order to gain more power, a supercharger or turbocharger can be used to push in more air. While a supercharger is belt-driven and moves air into the system based on engine RPM, a turbocharger is driven by exhaust gas, which turns a compressor at speeds that can exceed 100,000 RPM. This forces in additional air, and more air in the combustion chamber means more of the fuel can be burned at once.
The net effect of fuel injection and turbocharging is that a smaller engine can produce more power. A four-cylinder turbocharged direct injection engine can deliver as much power as a six-cylinder naturally aspirated engine – gaining up to 30 percent more torque, 25 percent better fuel economy and a 20 percent reduction in CO2 emissions versus conventional four-cylinder designs. Further, smaller engines reduce vehicle weight, adding to the fuel economy benefit. Combined with ultra-precise computer management, direct injection allows more accurate control over the amount of fuel injected (fuel metering) and exactly when the fuel is introduced into the cylinder (injection timing).
However, durability cannot be sacrificed at the altar of emissions and efficiency improvements. Lubricants help achieve the TGDI performance benefits expected by consumers while contributing to engine service life.
Consumer Demands
Bruce Belzowski, managing director of the automotive futures group at the University of Michigans Transportation Research Institute in Ann Arbor, noted during the webinar that future vehicle buying scenarios seem at odds with historic patterns. A survey of UM students showed that, save for price, the most important criteria for vehicle ownership are reliability and fuel economy.
The automotive industry expects TGDI engines to help meet these demands. A survey of powertrain experts (similar to ones conducted by UM as recently as 2012) found that all forms of powerplants – gasoline, hybrid, electric, diesel, fuel cell – in passenger cars and light trucks are predicted to grow through 2025. From 2020 on, however, spark-ignited engines will likely lose ground to other types, Belzowski reported. The same experts believe direct injection and turbocharging technologies are capable of improving fuel economy by up to 30 percent.
Geoff Duff, director of application engineering for North America at turbocharger producer Honeywell Transportation, expects 47 percent of engines produced globally will be turbocharged by 2020. That number will grow to 70 percent over the following 20 to 25 years. He pointed out that, of the fuel-saving technologies proposed for the industry, turbocharged engines are already in place.
Duff noted that fuel economy requirements globally are a major and enduring driver for turbocharged engines. These engines can do the job for about $60 more per percent of fuel economy increase, and deliver up to 25 percent improvement over current non-turbocharged engines.
Up to the Challenge
With new technology comes new problems that lubricants can help solve.
Probably the most well-known challenge for direct-injected engines is the phenomenon called low-speed pre-ignition (LSPI), or knock. This premature ignition of the main fuel charge is most common in certain TGDI engines operating in low-speed and high-load driving conditions, explained Tom Briggs, program manager for R&D of spark-ignited engines at Southwest Research Institute, the independent laboratory in San Antonio, Texas.
Knock has been characterized into four categories, he said: no knock; slight knock, which can sound like marbles rattling around and is sometimes called trace knock; heavy knock, which is a more discernable rumble; and super knock – the most damaging version. Super knock creates high in-cylinder pressure and has the potential to quickly cause serious damage to an engine.
Briggs believes that continued development of engines and lubricants is poised to solve the LSPI problem. Whats more, they must. Limits for LSPI are being introduced with the upcoming ILSAC GF-6 light-duty engine oil category, using a Ford-designed engine test that counts pre-ignitions.
According to Briggs, chain wear is also a serious problem for TGDI engines and has forced some OEMs to issue recalls. Southwest Research Institute believes timing-chain wear is related to lubricant degradation caused by small soot particle contamination. This issue is important enough that the new ILSAC GF-6 category will include a timing-chain wear test, too.
Intake port deposits are another concern, he continued. Deposits can be caused by fuel and engine oil from the positive crankcase ventilation system, backflow from the cylinder and leakage at the turbo compressor bearing seal. All lead to sticky deposit buildups, which some consumers have reported at relatively low mileages. This sludge can result in more frequent oil changes, excessive oil thickening and even engine damage.
Deposits, LSPI and wear all lead to shorter engine life, as well as poor fuel economy and increased service costs. Other challenges for oils in the era of TGDI, Duff noted, include temperatures up to 150 degrees Celsius, pressure decrease due to variable pumps, dilution with fuel (up to 15 percent in TGDI engines), and degradation, contamination and additive depletion. Since turbos are precision-made devices, repairs are expensive. With consumers continuing to demand durability in all conditions, new engine technologies cannot have a shorter lifespan than current systems.
On top of the inherent challenges of lubricating TGDI engines, Duff warned that some of the auto industrys goals foretell even greater demands on engine oils. For example, engine manufacturers are pushing for lighter oil viscosity grades in order to reduce friction in the engine, especially in cold conditions.
The Hunt for Solutions
Alex Sammut, global technology marketing manager at Lubrizol in Wickliffe, Ohio, pointed out that addressing lubricant issues such as sludge, wear and LSPI will speed the introduction and expansion of vehicle technologies like TGDI. Lubricant technologies, especially additive systems, advance on the back of new engine designs as well as attempts to address issues encountered in the field. Formulating involves an understanding of what these issues are and the parameters that define potential solutions.
Using the knowledge of which chemistries are responsible for various oil properties, additive formulators try to design a package that meets the needs of a specific engine. This may involve searching for the next generation of additive components. Once the proposed formulations are identified, researchers can begin testing them with bench and dynamometer evaluations.
In the next stage, Sammut continued, the proposed products are put to test in the field to determine whether or not proper performance has been achieved. Field testing verifies what works.
The lab and field results are then taken into the world of engine test development to be quantified and to set standards for lubricant performance. When that is done, the process starts all over with the next identified issue.
Sammut illustrated the process with the problem of LSPI. First, LSPI issues were reported in GDI vehicles in the field. Next, a consortium at Southwest Research Institute, along with other industry groups, identified the lubricants role in LSPI and suggested testing protocols. The American Petroleum Institute and OEM groups began including LSPI tests in their specifications, and a lubricant solution was identified to resolve the issue.
Deposits in turbocharged engines presented a significant challenge for oil formulators, he added. The industry worked to identify regions where fuel quality could be contributing to the issue, and a mainstream duty cycle pinpointed some unique lubricant challenges, as well. OEM-specified testing was developed and a lubricant solution was identified to keep engines clean, prolong service life and optimize performance.
As formulators addressed all of the identified problems, it was equally important to maintain and improve other characteristics of the lubricant. The bottom line is that the evolution of higher performance hardware always brings new challenges, Sammut concluded. These challenges can be overcome with the right chemistry and additive technology.
Consumers expect performance, reliability and fuel efficiency from new powertrain technology, summarized Lubrizol regional business manager Martin Birze, but new technology often brings unanticipated challenges. Investment in testing and R&D are required to solve these complex questions.
An integrated approach with all industry stakeholders is essential to create the next generation of lubricants that will enable future engine technologies. Lubricants can deliver benefits beyond their traditional role, Birze emphasized, and must be engineered around the needs of modern powertrain systems.